U.S. patent number 4,150,375 [Application Number 05/882,677] was granted by the patent office on 1979-04-17 for interferometric protective system for vehicles.
This patent grant is currently assigned to Sperry Rand Corporation. Invention is credited to Kenneth W. Robbins, Gerald F. Ross.
United States Patent |
4,150,375 |
Ross , et al. |
April 17, 1979 |
Interferometric protective system for vehicles
Abstract
A base band transmitter cooperates with dual base band receiver
antennas, a single receiver channel, and a tapped delay correlator
device for the prevention of vehicular collisions, the
configuration effectively narrowing the normally wide base band
antenna pattern to span a single predetermined forward traffic
lane.
Inventors: |
Ross; Gerald F. (Lexington,
MA), Robbins; Kenneth W. (Wilmington, MA) |
Assignee: |
Sperry Rand Corporation (New
York, NY)
|
Family
ID: |
25381107 |
Appl.
No.: |
05/882,677 |
Filed: |
March 2, 1978 |
Current U.S.
Class: |
342/21; 342/71;
342/93 |
Current CPC
Class: |
G01S
13/106 (20130101); G01S 13/931 (20130101); G01S
2013/93185 (20200101); G01S 13/0209 (20130101); G01S
7/2926 (20130101) |
Current International
Class: |
G01S
13/93 (20060101); G01S 13/00 (20060101); G01S
13/10 (20060101); G01S 7/292 (20060101); G01S
13/02 (20060101); G01S 009/04 () |
Field of
Search: |
;343/7VM |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hubler; Malcolm F.
Attorney, Agent or Firm: Terry; Howard P.
Claims
What is claimed is:
1. An apparatus employing reflection of base band electromagnetic
pulses from an object to be detected for activating vehicular
protective means in the nearby presence of said object comprising:
transmitter means for radiating base band electromagnetic pulses to
illuminate said object,
first and second receiver means spaced apart and symmetrically
disposed with respect to said transmitter means for receiving based
band electromagnetic pulses reflected from said object,
amplifier means,
switch means for alternately coupling said first and second
receiver means to said amplifier means,
tapped delay means having first and second opposed input means
respectively responsive to said amplifier means and to the
radiation of each base band electromagnetic pulse by said
transmitter means,
at least one shift register means responsive to a predetermined
tape of said tapped delay means intermediate said first and second
opposed input means thereof,
constant false alarm rate threshold means additionally responsive
to said amplifier means,
coincidence means responsive to simultaneous outputs of said
constant false alarm rate threshold means and said one shift
register means, and
actuator means responsive to said coincidence means.
2. Apparatus as described in claim 1 further including adjustable
delay means for delaying signals representing the radiation of each
base band electromagnetic pulse by said transmitter means before
application thereof to said second opposed input means.
3. Apparatus as described in claim 1 further including pulse
synchronizer means for operating said transmitter means at a first
rate and said switching means at a second rate, said first rate
being a harmonic of said second rate.
4. Apparatus as described in claim 2 further including range gate
generator means responsive to said adjustable delay means, said
constant false alarm rate threshold means having a gain responsive
to said range gate.
5. Apparatus as described in claim 4 further including threshold
limiter means in series connection with said adjustable delay means
and said second opposed input means.
6. Apparatus as described in claim 1 further including:
at least second shift register means responsive to a second tap of
said tapped delay means,
Or gate means responsive to said one and to said second shift
register means,
said coincidence means being additionally responsive to
simultaneous outputs of said constant false alarm rate threshold
means and of said second shift register means.
7. Apparatus as described in claim 1 wherein said tapped delay
means includes a plurality of taps substantially equally spaced by
an amount equal to one half the product of the signal propagation
velocity in said tapped delay means and the duration of said base
band electromagnetic pulses transmitted by said transmitter
means.
8. Apparatus as described in claim 7 wherein said tapped delay
means comprises transmission line means having one tap centrally
disposed between said first and second opposed input means.
9. Apparatus as described in claim 1 wherein said actuator means
comprises means for protective operation of the brakes of a
vehicle.
10. Apparatus as described in claim 1 wherein said actuator means
comprises means for warning the operator of a vehicle of the nearby
presence of said object.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to base band presence detection and range
measurement systems and more particularly to means effectively
narrowing the antenna patterns of such base band radio systems as
employed in vehicular protective systems.
2. Description of the Prior Art
Systems employing sub-nanosecond base band pulses as object
detectors generally exhibit beam widths too broad for good
discrimination of angularly displaced targets. Good angular
resolution is achieved in the more conventional pulse radar systems
by the use of highly directive antennas. However, such antennas
have relatively narrow pass bands. Antennas that are broad band
enough to propagate sub-nanosecond pulses also have an inherently
broad beam width. Where it is desired to detect targets within
about two meters of the detector, the inherent broad beam widths
produced by base band antennas are not a particular problem because
the outer boundary of the range gate and the natural fall off of
the antenna response severely restrict the area covered at the
short range. For some distances beyond this range, signal processng
techniques have been successfully employed to narrow the effective
beam widths. For example, one such technique has been described in
the U.S. Pat. No. 3,858,205 to G. F. Ross for a "Base-Band
Precollision Sensor with Time Domain Gating for Preventing False
Responses", issued Dec. 31, 1974 and assigned to Sperry Rand
Corporation. However, such techniques become increasingly
ineffective as the range to the target is greatly increased with
respect to the separation of the transmitter and receiver
antennas.
Generally speaking, it is difficult to obtain a system that
exhibits both a narrow effective beam width and negligible signal
dispersion (i.e., has a wide band width). In antenna array systems,
for example, one can obtain a narrow beam by employing a
multiplicity of elements and by making the aperture dimensions
large compared to the center wave length. However, to obtain
simultaneously a wide band width, it is necessary to employ real
time delays in the element feed network rather than instantaneous
phase shift behind each element to afford coherent addition at the
appropriate sum port.
A further solution of the problem is set forth in the G. F. Ross
U.S. Pat. No. 4,017,854, issued Apr. 12, 1977 for "Apparatus for
Angular Measurement and Beam Forming With Baseband Radar Systems",
assigned to Sperry Rand Corporation. In this latter system, there
is described an electromagnetic energy pulse system for
transmitting very short base band pulses and for the reception of
such pulses by dual base band receivers, the receiving antennas
being separated by a distance much less than the range to the
target to be detected. The base band receivers amplify the received
signals, the amplified signals being further processed by being fed
to opposite ends of a tapped transmission line, whereupon the angle
to the target may be determined by the tap at which the two pulses
arrive in coincidence. The taps are spaced along the transmission
line, whereupon the angle to the target may be determined by the
tap at which the two pulses arrive in coincidence. The taps are
spaced along the transmission line by distances somewhat greater
than one-half the product of the signal velocity on the
transmission line and the duration of the received and
reconstituted pulses, thus providing a means by which the relative
time delays between the two receiving antennas may be determined to
within one half the pulse width and concomitantly establishing an
effective radiant beam width for the system proportional to the
ratio of the pulse duration to the distance separating the two
receiver antennas. While representing a valuable improvement over
the prior art, the system of U.S. Pat. No. 4,017,854 has certain
difficulties that will be discussed in more detail hereinafter,
especially when used for automatic braking in road vehicles. Such
problems make the prior art devices generally unsuited for use, for
example, for operating vehicular braking mechanisms with respect to
roadway lane widths of about 2.0 meters at a 45 meter forward
distance of the intruding object, since effective antenna pattern
widths as narrow as 2.7 degrees are then needed. In such an
example, symmetrically disposed receivers spaced by about 0.75
meters from the transmitter would be employed.
SUMMARY OF THE INVENTION
The present invention relates to electromagnetic base band pulse
train transmitter and symmetrically disposed dual receiver antenna
systems in which the cooperating receiver antennas are placed
symmetrically about the transmitter antenna and cooperate with a
single base band receiver. The configuration, as applied in the
field of vehicle collision avoidance and braking systems, employs a
tapped delay correlation device, the arrangement effectively
improving the resolution of the interferometer device, narrowing
the normally wide associated antenna patterns so as to span only a
single predetermined forward traffic lane. System cost and
correlation efficiency are enhanced by the use of the single
channel receiver cooperating with the receiver antenna pair.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the prior art system, showing its
components, their electrical connections, and the geometrical
relationship of the target transmitter and receivers.
FIG. 2 is a block diagram showing a first feature of the present
invention.
FIG. 2a provides wave form graphs useful in explaining the
operation of the invention.
FIG. 3 is a block diagram showing a second feature of the present
invention.
FIG. 4 is a block diagram of a system alternative to that of FIG.
3.
FIG. 5 is a block diagram of a variation of the embodiment of FIG.
4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the base band pulse transmitter 10 employed
therein is actuated by a conventional current pulse generator 11
also normally supplying range gate synchronizing signals for the
control of the base band pulse receivers 12 and 13. Transmitter 10
provides base band pulse signals for forward radiation by antenna
14. Signals reflected by the surface 15 of an object are collected
by receiver antennas 16 and 17 for supply to receivers 12 and 13,
respectively, receiver antennas 16 and 17 being symmetrically
located on either side of transmitter antenna 14 and separated a
distance d therefrom, the distance d being much shorter than the
length of ray path 34 from antenna 14 to the reflecting surface 15.
Ray path 34 is shown at an arbitrary angle .theta. from the
perpendicular 18 to the base line 21, which latter passes through
antennas 14, 16, and 17, while ray path 34a between receiving
antenna 16 and surface 15 and ray path 34b between receiving
antenna 17 and surface 15 form respective angles .theta.' and
.theta." with perpendiculars 18a and 18b. It will be understood
that the geometry of FIG. 1 is distorted for the sake of clarity.
Though not apparent in FIG. 1, ray paths 34, 34a, and 34b are
normally much longer than d. Thus, the respective angles .theta.
and .theta." which ray paths 34a and 34b form with respect to
perpendiculars 18a and 18b are approximately equal to .theta.,
making the three ray paths 34, 34a, and 34b nearly parallel.
Transmitter 10 and its associated antenna 14 may take any of
several forms, descriptions of suitable combinations being given in
U.S. Pat. No. 3,659,203, issued Apr. 25, 1972 to G. F. Ross and D.
Lamensdorf for a "Balanced Radiator System" and in the U.S. Pat.
No. 3,728,632 issued Apr. 17, 1973 to G. F. Ross for "Transmission
and Reception System for Generating and Receiving Base-Band
Duration Pulse Signals Without Distortion for Short Base Band Pulse
Communication System".
Each of receivers 12 and 13 may be base band receivers of the
general kind described in U.S. Pat. No. 3,662,316, issued May 9,
1972 to K. W. Robbins for "Short Base-Band Pulse Receiver" or in
U.S. Pat. No. 3,728,632. All of the aforementioned patents are
assigned to Sperry Rand Corporation. The two outputs of receivers
12, 13 on the respective leads 22, 23, which outputs may be
reconstructed video pulses derived from the pulses received at
receiver antennas 16 and 17, may be passed through respective
manually or otherwise adjustable compensating delay devices found
within signal processor 24 for purposes explained in the
aforementioned U.S. Pat. No. 4,017,854 before processing in device
24 for correlation or other purposes. In the prior art device, the
correlator 24 includes a transmission line with a number of equally
spaced taps located thereon. One end of the transmission line is
coupled to the output terminal of a first variable delay and the
other end is coupled to the output terminal of a second variable
delay, whereby the output pulses from the variable delays are
coupled to the transmission line so that they propagate therein in
opposite directions but toward each other. Connected at each tap of
the transmission line is a coincident detector comprising a diode
biased at a threshold voltage which may be coupled to a display or
other utilization device.
Reconstituted video pulses coupled to the transmission line are of
essentially equal duration and amplitude. When the pulses are
simultaneously launched in the transmission line with a pulse width
that is very much less than the transit time across the
transmission line, the two pulses meet only in the vicinity of the
midpoint of the line. At this central tap, a pulse will form with
an amplitude equal to twice the amplitude of each of the input
pulses and with a pulse duration equal to the pulse width of each
of the launched pulses. As at 79 in FIG. 3, the prior art
transmission line also possesses a center tap 0 and taps on either
side thereof. Taps to the left of the center tap 0 are numbered
with positive integers and those to the right with negative
integers. If the output pulse at one end of the transmisson line is
delayed relative to the input pulse at the other being launched on
the transmission line, the double amplitude pulse will form at one
of the positively numbered taps whereas, if the input pulse at one
end of the transmission line is launched on the transmission line
with a delay relative to the other input pulse, the double
amplitude pulse will form at one of the negatively numbered taps.
When the pulses coincide at a given tap, they are coupled to an
associated detector circuit which provides an indication that the
pulses received at antennas 16 and 17 have the time relationship
represented by the given tap. To prevent conduction when the two
pulses do not coincide, the detector is suitably back biased.
In the improvement of FIG. 2, the prior concept portrayed in FIG. 1
is modified to adapt it more adequately to perform the vehicle
braking function. Elements common to the prior art system of FIG. 1
bear the same reference numerals in FIG. 2. In place of the two
independent receivers 12 and 13 of FIG. 1, the configuration of
FIG. 2 is modified to use only one receiver channel comprising the
series connected switch 50, wide band amplifier 52, and constant
false alarm rate threshold device 54. Switch 50 may be a
conventional internally electrically actuated, rapid operating
semiconductor device having a wide pass band with a minimum loss
and delay. Threshold device 54 may be generally similar to
arrangements disclosed by A. M. Nicolson and R. J. Brophy in the
U.S. Pat. No. 3,755,696 for a "Detector Having A constant False
Alarm Rate and Method for Providing Same", issued Aug. 28, 1973, or
in the A. M. Nicolson and R. M. Mara U.S. Pat. No. 3,983,422 for a
"Detector Having a Constant False Alarm Rate", issued Sept. 28,
1976, both patents being assigned to Sperry Rand Corporation. Such
constant false alarm rate threshold circuits may consist of
threshold detectors including a bistable device such as a tunnel
diode with a first low-voltage stable state and a second
high-voltage stable state and a variable threshold level sensitive
to power supply fluctuations and to both noise signals and to
useful input signals. Circuits for determining an active range
gating interval for controlling the sensitivity of the bistable
device are included, as well as counter circuits for separately
indicating the presence of a target and for maintaining constant
the false alarm rate in the detector circuit.
Transmitter 10 is synchronized by a pulse synchronizer 44 which may
be crystal controlled and which also controls other features of the
FIG. 2 system, as indicated in FIG. 2a by way of example. Pulse
synchronizer 44 controls the timing of frequency divider 42 via
electrical lead 43. The frequency divider or equivalent device 42
is arranged to generate at least three pulse trains f.sub.1,
f.sub.2, f.sub.3 found respectively on leads associated with
junctions 39, 40, and 41. The f.sub.2 pulse train shown as pulses
66 in FIG. 2a is used to excite transmitter 10 which, in turn,
radiates a corresponding base band pulse train via antenna 14.
In order to assure that the novel signal processor system 24' is
accurately synchronized with the actual pulses as they are emitted
by antenna 14 toward the reflecting object surface 15, capacitively
coupled versions of the emitted pulses are supplied via lead 56 to
variable delay 57 for dual purposes. Variable delay device 57 may
be a multiple turn potentiometer calibrated in increments of range
delay. On the other hand, a conventional electronically shiftable
delay device may be employed. The output of variable delay 57 is
coupled via leads 58, 58a to a conventional range gate generator
60, such as the range gate generator of the general type, for
example, disclosed in the Ross U.S. Pat. No. 3,750,025 for an
"Energy Amplifying Selector Gate for Base-Band Signals", issued
July 31, 1973 and assigned to Sperry Rand Corporation. Range gate
generator 60 thus yields a gate positioned according to the setting
of delay 57 supplied via lead 55 for the control of the constant
false alarm rate threshold device 54. A second control signal
supplied via terminal 39 to constant false alarm rate device 54 is
the pulse train of frequency f.sub.1 generated by divider 42. The
range gate pulses on lead 55 are applied to C.F.A.R. threshold
device 54; thus, there is a range gate for the reception of each
target echo. The C.F.A.R. threshold device 54 automatically adjusts
itself in a conventional fashion to its maximum sensitivity because
a range gate is generated when the transmitter is off.
In this manner, signals from the threshold device 54 which
represent only true echo responses received by antennas 16, 17 are
supplied through a single receiver channel to the input 63 of
signal processor 24'. These pulse train signals are therein
correlated in a novel manner with respect to second pulse train
signals from transmitter 10 appearing on lead 58b at the output of
variable delay 57 and passing through the conventional threshold
unit 61. It will be understood that the threshold levels of devices
54 and 61 are conveniently adjusted to pass only pulses of a
sufficient amplitude that the signals on input leads 62 and 63 of
signal processor 24' respectively represent true received signals
and true transmitted signals rather than spurious signals.
In the particular application of the invention to vehicular
braking, unavoidable variations of the effective delays within the
dual channel system of FIG. 1 are difficult to tolerate. The
meaningful differential delays of paths 34a, 34b that it is desired
to measure are defined in inches (corresponding to a duration of
several hundred picoseconds). The use of dual channels can not be
allowed in the particular application at hand, the possible maximum
drift being about the same as the meaningful value to be measured,
or even greater. The system of FIG. 2 removes the drift problem,
switch 50 being used in a single receiver amplifier channel, a
relatively less expensive arrangement than that of FIG. 1. The
system of FIG. 2 is also readily adapted to use with the simple
signal processor devices 24' which will now be considered with
respect to FIGS. 3 and 4.
In the embodiment of FIG. 3, elements common to those of the FIG. 1
and 2 arrangements again bear similar reference numerals. It will
also be assumed that transmitter 10 is again driven synchronously
by the pulse signal train of frequency f.sub.2 at junction 40 of
FIG. 2, and that switch 50 and the constant false alarm rate device
54 are also controlled by the f.sub.3 and f.sub.1 pulse trains
appearing in FIG. 2.
As in FIG. 2, the configuration of FIG. 3 uses a transmitter 10
with an associated radiating antenna 14, two receiver antennas 16,
17 spaced equally from radiating antenna 14, and a single receiver
channel in which outputs from receiver antennas 16, 17 are
successively switched by switch 50 into a single wide band
amplifier 77. It will be understood that switch 50, though shown in
the several figures for simplicity as a purely mechanical switch,
will preferably be a semiconductor or other fast operating
electrically driven switch operated as in FIG. 2a.
The novel signal processor 24' found in the lower part of FIG. 2 is
built around a conventional delay or delay line device 79 as will
be discussed in further detail with respect to FIG. 3. It is to be
understood with respect to FIG. 3 that the operation of switch 50
with respect to antennas 16, 17 and to the driver signals on
terminal 41 is generally the same as in FIG. 2, as is the operation
of base band pulse transmitter 10 with respect to the synchronizing
pulse train placed on terminal 40.
When the video pulses flowing into delay device 79 at inputs 76 and
58 arrive simultaneously at the same tap of the array of taps
including the tap +4, a corresponding double amplitude pulse is
coupled out of that tap into one shift register such as register
81. For further example, if the incoming pulses arrive
simultaneously at the ends of the uniform delay device 79, they
will inherently arrive at the 0 tap simultaneously to yield a
double amplitude pulse flowing into and operating the central shift
register 82. This latter situation obtains when line 34 in FIG. 1
lies on top of the boresight line 18 and lines 34a, 34b are of
equal length, angle .theta.' equals angle .theta.", and angle
.theta. is zero. Simultaneous arrival of pulses at taps other than
that feeding shift register 82 occur for finite positive or
negative values of .theta..
In FIG. 3, the output of amplifier 77 is fed via lead 53 to a
conventional constant false alarm rate threshold circuit 54 such as
disclosed in the aforementioned patents and whose operation is
additionally controlled by the f.sub.1 pulse train appearing on
junction 39 of FIG. 2. Device 54 yields a pulse representing a true
target on output lead 85. Any output from delay device 79 which, as
has been shown, results from the presence of true pulses
representing an actually transmitted pulse and a true received echo
pulse, is fed through one of the shift registers, 80, 82, et
cetera, and ultimately arrives at OR gate 86.
The verified signals on the respective output leads 85 and 91 of
device 54 and of OR gate 86 are supplied to AND gate 87. If both
are present simultaneously in AND gate 87, a verified output
reliably appears on lead 88 for use as desired. For example, the
output signal may be used for control purposes by actuator 90 or by
other utilization means. Alternatively or additionally, the signal
lead 88 may be coupled to actuate a warning device 89, audible or
visual, for alerting the vehicle operator.
In a typical arrangement of the FIG. 3 apparatus, four transmitter
pulses are transmitted during the dwell of switch 50 on each of its
input terminals, as seen in FIG. 2a. Accordingly, eight pulses are
stored in a particular shift register while .theta. is constant.
Thus, the registers 81, 82, et cetera, are eight-bit registers
recycled every eight target hits, one each of which is completely
filled on receipt of eight echo pulses. When .theta. stays
constant, a useful output signal verifying the presence of a target
is transmitted to OR gate 86. Its output on lead 91 inhibits
steppable delay 57, thus causing the range gate to remain fixed.
When twenty six out of thirty two consecutive bits are detected,
for example, the C.F.A.R. conventional summation signal on lead 85
and the register signal 91 enable AND gate 87 and hence devices 89
and 90. If .theta. changes because of relative motion between the
target direction 34 and the boresight line 18, pulses are stored in
a corresponding shift register, the next reset pulse on terminal 41
from frequency divider 42 (FIG. 2) recycling the register array,
also wiping out the data in those only partially filled.
In the FIG. 2 and 3 systems, the plurality of taps of delay device
79 readily renders the system useful for a variety of target ranges
selectable by the vehicle operator. However, the less expensive
system shown in part in FIG. 4 may be preferred where actuation is
generally desired at a prescribed target range. This simplified
system requires a delay device 79' having only one centrally
located tap cooperating with a single central eight-bit shift
register 82' regularly recycled by reset pulses applied by terminal
41. The output of the eight-bit register 82' is fed to AND gate 87
together with the conventional sum output at 85 of the constant
false alarm rate device 54. Thus, when a target is within the range
gate as defined by the setting of delay 57 (at 20 feet, for
instance), and its echo is of sufficient intensity for more than
twenty-six out of thirty-two consecutive transmissions, it produces
an input on lead 85 to AND gate 87. An output from both device 54
and the eight-bit register 82' produces an alert signal in warning
device 89 or an actuation of actuator 90 via AND gate 87. Inputs
110, 111, 112, 113, 114 correspond to the similarly numbered inputs
of FIG. 3.
In the embodiment of FIG. 5, which is generally similar to that of
FIG. 4 in that a single shift register 82 is used in the processor
24', elements common to preceding figures again bear similar
reference numerals. The FIG. 5 system differs principally in the
manner in which the output signals of the single shift register 82
are handled, the output of AND gate 87 being supplied to two
additional elements, the step programmer 100 and counter 101. The
AND gate 87 output, signalling a proper response to a real target,
enables the conventional step program circuit 100 which, in turn,
inhibits the control signal on lead 91. The step programmer circuit
100 now causes the conventional steppable delay 57 to reposition
the range gate to a closer range and to hold the gate in its new
position. If AND gate 87 once again signals a response to a real
target, the step programmer 100 again moves the range gate
incrementally to a lower range. This operation may be continued for
as many interrogations as are pre-programmed by counter 101. For
example, if the counter 101 is programmed for a predetermined
number such as three, and three successful interrogations have been
indicated as successive outputs of AND gate 87, the output of
counter 101 then enables the warning and actuator devices 89, 90.
Whenever there are fewer successful interrogations than the
predetermined number, the step programmer circuit 100 reverts to
normalcy and lead 91 is again enabled, counter 101 being reset at
the same time.
With respect to FIGS. 1 through 5 and to the teachings of the G. F.
Ross U.S. Pat. No. 4,017,854, an analysis appearing in the latter
will be instructive. Assume in FIG. 1 that each of receiver
antennas 16 and 17 has received a reflected pulse from an object
surface 15, the object surface having been illuminated by a pulse
radiated from the transmitter antenna 14. The reflecting surface 15
is located at the intersection of a first ellipse defined by the
spacing between the receiver antenna 16 and the transmitter antenna
14, which are disposed at the foci of the ellipse, and the total
path length from the transmitting antenna 14 to the object surface
15 and from the object surface 15 to the receiver antenna 16, a
second ellipse being similarly defined by the receiver antenna 17
and the transmitter antenna 14. Since the separation d between
transmitter antenna 14 and receiver antennas 16 and 17 is much less
than the range to the reflecting surface 15, the ray path 34 which
forms an angle .theta. with the perpendicular 18 from the
transmitter antenna 14 to the reflecting surface 15 is nearly
parallel to the ray paths 34a and 34b from the reflecting object to
the receiver antennas 16 and 17, respectively, as previously
discussed. The angles .theta.' and .theta." in FIG. 1 that these
ray paths make with the perpendiculars 18a and 18b are
approximately equal to angle .theta.. Thus, the path length
difference P between the ray paths 34a and 34b, as determined by
the perpendicular 35 from the receiving antenna 17 to the ray path
34a, is given by P= 2d sin .theta., whereby the differential time
delay T between reception at antenna 17 and reception at antenna 16
is P/c, where c is the signal velocity in free space. Assume that
the tap spacing S, as shown in FIG. 3, is sufficient for the
resolution of two pulses, i.e., is equal to one-half the distance
that a signal with signal velocity v along transmission line 79,
travels during the pulse width interval .tau.. Assume also that ns
is the distance from the center of the transmission line 79 to the
tap at which the two pulses, launched from opposite ends of the
transmission line 79, coincide. Where n is the tap number which can
take on values of 0, +1, .+-.2 . . . +N, it is apparent that:
or:
Thus, the tap at which the two pulses launched on transmission line
79 coalesce is determined by the angular location from the
boresight 18 to the reflecting surface 15.
Again, as taught in the aforementioned U.S. Pat. No. 4,017,854,
median intersecting ellipses may be drawn through a particular
point target on surface 15, each of which may be bounded by
ellipses that define the range resolution 2.delta. of the
transmitter antenna 14-receiver antenna 16 and transmitter antenna
14-receiver antenna 17 combinations. These bounding ellipses define
a resolution cell as explained in U.S. Pat. No. 4,017,854 with an
angular resolution of:
and a range resolution given by:
whereby, for small angles of the reflecting surface 15 with respect
to boresight 18, angular resolution is approximately
The angular region of detection may be limited by restricting the
length of transmission line 79 or 79' or the number of taps
thereon. Thus, transmission line 79 or 79' with a single centrally
located tap as in FIG. 4 restricts the angular coverage to the
resolution cell about boresight. Beam scanning to either side of
the boresight direction may be accomplished with this arrangement
by manually or otherwise adjusting variable delay 57 which
previously has been adjusted only to compensate for differential
time delays inherent in the receiver and antenna combinations. The
angle through which the beam is scanned by this procedure is defind
by:
where .tau. is the differential time delay inserted in delay
57.
While the invention has been described in its preferred embodiment,
it is to be understood that the words which have been used are
words of description rather than of limitation and that changes may
be made within the purview of the appended claims without departing
from the true scope and spirit of the invention in its broader
aspects.
* * * * *